In cardiomyocytes L-type CaV1.2 Ca2+ channels mediate the ultimate cellular response to electrical activity: a change in intracellular Ca2+. The essential CaV1.2 channels are subject to multiple control mechanisms that when perturbed cause arrhythmias and heart failure. In this application we build upon our previous studies to address new aspects of CaV1.2 regulation and dysregulation with the overarching goal of exploring how fibroblast growth factor homologous factors (FHFs) confer previously unrecognized regulation upon CaV1.2. A deceptively named subgroup of fibroblast growth factors (FGF11-14), FHFs are neither secreted nor function as growth factors. Rather, FHFs remain intracellular and were discovered to modulate voltage-gated Na+ channels. Here, we focus on our new findings, which reveal that FHFs are unexpected components of macromolecular complexes that influence CaV1.2, to explore CaV1.2 regulation and dysregulation by FHFs. We propose: 1) To determine how FHFs regulates CaV1.2 channel trafficking and targeting in cardiomyocytes. This builds on our preliminary data showing that FHFs affect trafficking and targeting of CaV1.2 to T-tubules in cardiomyocytes; and upon our discovery of specific FHF binding partners that may regulate these processes. We will exploit a new rapid knockin method to test informative mutants in cardiomyocytes. 2) To determine how FGF13 affects function and dysfunction within the heart. Here, we build upon new data showing that FGF13, the dominant FHF in rodent heart, regulates not only CaV1.2 function, but consequences of CaV1.2 function, such as the intracellular Ca2+ transient and the ventricular action potential. We have developed new mouse models to determine how FGF13 affects cardiac function at baseline and at stress. 3) To analyze the consequences of mutations in human FGF12. We have identified several mutations in FGF12, the FHF dominant in human heart, in patients with arrhythmia syndromes. We propose to examine how these mutations lead to arrhythmias using the novel and validated knock-in method in ventricular cardiomyocytes.